From what I can see this release rev15 may allow DVD's to be used, but makes neogamma inoperable. I think people have been messing aroung with it and other IOS trying to get NSMB to work. It may end like rev13 and 13b, where you decide what features you want.
Actually getting NSMB to work was quite simple, if you use a USB loader, myself, davepm and many others had it working within an hour, on USB HDD.
I made a channel wad for the game to help people, but still the thread with the excellent guide by davepm grows.

yes i had instant success with the guide as well and was trying to help ppl that for the life of me ,i can't understand why refused to read anything at all. i was'nt trying to get it to run from disk, but when a few folks mentioned they did using rev 15 i had to know why they could use it and i couldn't. and i guess the difference was the dvd+r issue. since i have no desire to convert my collection of dvd-r games over to dvd+r and nsmb works fine for me on sd card in the first try. i overwrote the file with rev14. i sure hope it will be ok i haven't noticed any issues but from what some ppl are saying who knows.

PAL/4_2E/D2E dvdrom/Cios38R15/Cios222 and 223/Cioscorp3.5 Wii here. The new Cios38R15 seems to fix NSMB from copy unpatched disc. I use NeogammaR8B7 to boot from unpatched PAL disc. I use Configurable USBLoader V49 to boot from HDD. I didn't play it long enough to notice any probs from HDD with REV14 or REV15. But REV15 fixed it to boot from disc from Neogamma. REV14 not boot from disc period for me.

I thinks those who are having probs with REV15 maybe used IOS249 to install. I used patched IOS36 to install REV15. Works nice. No complain here. World 6-2 Have about 15 customers running REV15 also. No complains Also SSBB works from copy disc and HDD using 222-mload in USBLoader + BCA dump.

I'm admittedly a little dumb when it comes to all of these cios revisions - do they run side-by-side and you can choose which to run as (like the IOS in the USB Loader GX options), or do you decide to install one OR another?

They don't run side by side, you just choose which verson you want to use, you can quite happily change to whatever version you want without uninstalling as the the new version simply overwrites the old one.